In recent years, it was reported that a semiconductor device using an Al.sub.x Ga.sub.y In.sub.z N (where 0.ltoreq.x.ltoreq.1, 0.ltoreq.y.ltoreq.1, 0.ltoreq.z.ltoreq.1) type compound semiconductor, which is a III-V-group compound semiconductor containing nitrogen as a V-group element, has outstanding light-emitting characteristics at room temperature. This type of compound semiconductor has been expected to be a material for realizing a blue light-emitting device (e.g., Japanese Journal of Applied Physics 32 (1993) L8-L11). Semiconductor devices having such a gallium nitride type compound semiconductor are obtained by epitaxially growing an n-type semiconductor layer, an i-type semiconductor layer, or a p-type semiconductor layer made of Al.sub.x Ga.sub.1-x N and In.sub.y Ga.sub.1-y N (where 0.ltoreq.x.ltoreq.1, 0.ltoreq.y.ltoreq.1) on a sapphire substrate.
As a method of epitaxial growth, a Metalorganic Vapor Phase Epitaxy (MOVPE) method, a Molecular Beam Epitaxy (MBE) method, and the like are generally used. For example, the MOVPE method will be briefly described. Organic metal such as trimethylgallium (TMGa; Ga(CH.sub.3).sub.3), trimethylaluminum (TMAl; Al(CH.sub.3).sub.3) and trimethylindium (TMIn; In(CH.sub.3).sub.3) and ammonia (NH.sub.3) gas are supplied as a reaction gas into a reaction chamber in which a sapphire substrate is provided. An AlGaInN type compound semiconductor is epitaxially grown on the sapphire substrate while the surface temperature of the substrate is kept at a high temperature of 700 to 1100.degree. C. At this time, the AlGaInN type compound semiconductor can be controlled so as to have p-type conductivity, i-type conductivity, or n-type conductivity by supplying diethylzinc (DEZn; Zn(C.sub.2 H.sub.5).sub.2), silane (SiH.sub.4) or the like to the reaction chamber.
As a known typical device using an AlGaInN type compound semiconductor, there is a blue light-emitting diode (e.g., Japanese Journal of Applied Physics 32 (1993) L8-L11). This light-emitting diode has a double hetero structure including an active layer made of In.sub.0.05 Ga.sub.0.95 N, to which zinc (Zn) is added. This is because the light-emission wavelength is adjusted to about 0.45 .mu.m. In this manner, only zinc is added to an active layer in the conventional light-emitting diode using an InGaN active layer.
Furthermore, a light-emitting diode having an MIS structure including an active layer made of Al.sub.x Ga.sub.1-x N is reported (Japanese Laid-Open Patent Publication Nos. 4-10665, 4-10666, and 4-10667). In this light-emitting diode, Zn and Si are simultaneously added to an i-type GaN which is to form a light-emitting layer, so as to change the wavelength of the emitted light.
However, these conventional light-emitting diodes have the following problems:
In the case where only Zn is added to an In.sub.y Ga.sub.1-y N layer, a Zn atom pairs off with an intrinsic defect such as vacancy in the In.sub.y Ga.sub.1-y N layer to be stabilized. However, this decreases the emission efficiency because the intrinsic defect becomes a non-radiative center. In addition, the intrinsic defect increases under the application of an electric field, causing adverse effects such as an increased driving current and decreased reliability of the light-emitting device. In particular, these problems become serious in semiconductor lasers requiring high optical density and carrier density. Furthermore, in the case of using Zn as a p-type impurity, a large amount of Zn should be added because of its low activation ratio.
In order to increase the light-emission wavelength, an increase in the In composition of In.sub.y Ga.sub.1-y N used for an active layer is required. For example, the wavelength is desirably in a blue-green region, i.e., about 520 nm in the case of traffic lights and the like. For this purpose, the amount of In in an In.sub.y Ga.sub.1-y N active layer is required to increase. However, the increase in amount of In decreases the quality of crystal, resulting in the decrease in emission efficiency. Thus, it is difficult to obtain a light-emitting device emitting light having a long wavelength by increasing a composition ratio (mole fraction) of In.
In the case where an AlGaN layer is used as an active layer, its bandgap energy is 3.44 eV or more at room temperature, that is, the wavelength of the emitted light is 360 nm or less. Therefore, even when zinc and silicon are simultaneously added in large amounts, the light-emission wavelength is not in a blue region. Thus, it is very difficult to obtain a blue light-emitting device by using an AlGaN layer as an active layer. The AlGaN layer also has a problem in that a deep impurity level is likely to be formed, causing unnecessary light emitted through the deep impurity level; thereby increasing a working current of the light-emitting device.
III-group nitrides, such as Al.sub.x Ga.sub.1-x N and In.sub.y Ga.sub.1-y N, are stable materials which are not subject to corrosion by chemical agents and are difficult to etch. Thus, it is difficult to form a structure for confining carriers in an active layer in a light-emitting device such as a semiconductor laser.
Furthermore, it is difficult to form a cavity of a semiconductor laser by cleavage because a sapphire substrate is used in a GaN type light-emitting device. The formation of a cavity can be considered to be done by dry etching such as reactive ion etching, ion milling, and focused ion beam etching. In this case, defects are formed in a semiconductor layer in the vicinity of facets of the cavity. These defects cause problems similar to those caused by the above-mentioned intrinsic defect.
In a semiconductor light-emitting device using an insulating substrate such as a sapphire substrate, electrodes cannot be provided on a substrate side. Therefore, a step difference is provided in a part of a semiconductor layer forming the light-emitting device, and electrodes are required to be respectively provided on a convex portion and a concave portion formed by the step difference. In the case of a GaN type light-emitting device, a step difference of 1 .mu.m or more is required. However, in this case, it becomes difficult to allow a semiconductor layered structure forming a semiconductor light-emitting device to adhere to a heat sink having good heat conduction. In the case where heat-releasing of a semiconductor light-emitting device having a step difference is imperfect, there are problems in that a working current increases along with the increase in temperature of the device and reliability of the device decreases. Furthermore, when excess stress is applied to an active layer while the semiconductor layered structure forming the semiconductor light-emitting device is allowed to adhere to the heat sink, there is a possibility that light-emission stops. Even in the case where the amount of stress is small, there is a problem in that birefringence is caused in materials forming the semiconductor layered structure, and plane of polarization of light emitted from the semiconductor light-emitting device rotates. For example, this causes inconvenience in the case where a semiconductor laser is used for an optical disk or the like.
The present invention solves the above-mentioned problems in the prior art and provides a highly reliable light-emitting device capable of emitting light with a wavelength in the blue region at high emission efficiency. Moreover, the present invention provides a semiconductor light-emitting device which has an outstanding heat-releasing property and in which there is less rotation of the plane of polarization of emitted light. Furthermore, the present invention provides a method for producing such a blue light-emitting device in simple steps with high yield.